Described herein is an electrophoretic display device. More particularly, described is a full color electrophoretic display device having a novel structure that permits the display to exhibit truer color with good brightness while utilizing a straightforward one or two color particle display system.
The electrophoretic display devices herein are thus capable of generating images, including full color images. The electrophoretic displays herein may be used for any display application, and particularly any display application where the image displayed may be changed, including, for example, reimageable paper, electronic books, electronic signage, watch, monitor and/or cell phone displays, and the like.
Electrophoretic displays are well known in the art. An electrophoretic display generally comprises a suspension of one or two charged pigment particles colloidally dispersed in a clear or colored liquid of matching specific gravity and contained in a cell comprising two parallel and transparent conducting electrode panels. The charged particles are transported between the electrode panels under the influence of an electric field, and can therefore be made to display an image through appropriate application of the electric field on the electrodes. The advantages of electrophoretic displays as a means for providing information and displaying images has been well appreciated.
U.S. Pat. No. 4,272,596, incorporated herein by reference in its entirety illustrates an electrophoretic display structure. The electrophoretic display device comprises a white marking material such as titanium dioxide suspended in a colloidal dispersion containing black colorant such as colloidal size iron oxide particles known as ferrofluids.
U.S. Pat. No. 6,113,810, incorporated herein by reference in its entirety, describes a dielectric dispersion for use in an electrophoretic display that includes a dielectric fluid, a first plurality of particles of a first color having a surface charge of a selected polarity dispersed within the dielectric fluid and a second plurality of particles of a second color having a surface charge of opposite polarity to that of the first plurality and a steric repulsion thereto preventing coagulation of the first and second plurality of particles.
U.S. Pat. No. 6,017,584, incorporated herein by reference in its entirety, discloses encapsulated electrophoretic displays in which particles encapsulated therein are dispersed within a suspending, or electrophoretic, fluid.
U.S. Pat. Nos. 6,525,866 and 6,577,433, each incorporated herein by reference in its entirety, disclose an electrophoretic display liquid composition for use in an electrophoretic display device that has a multiplicity of individual reservoirs, each containing a display liquid comprised of two sets of particles dispersed in a transparent liquid system.
Electrophoretic display is thus based on the migration of charged particles suspended in an insulating fluid under the influence of an electric field. As discussed above, the display systems may comprise one-particle and two-particle systems. One-particle systems typically comprise one set of same color particles dispersed in a fluid that may have a color different from that of the particles. Two-particle systems typically comprise two sets of colored particles, with each set of particles having a different color, dispersed in a fluid that is typically colorless/clear. A downside to these conventional one-particle and two-particle systems has been the difficulty in achieving full color displays using such systems.
Up to this point, it has been believed that the path to reflective colored electrophoretic displays would require significant modification of the traditional one-particle and two-particle display systems. This is because such systems could at best achieve only two colors.
Current approaches for color devices involve the use of color filter overlays on black and while two-particle display systems, or the use of lateral particle migration of monochrome particles with colored backplanes (referred to as shuttered displays).
An example of an electrophoretic display with a color filter is U.S. Pat. No. 6,850,355, describing an electrophoretic display comprising a transparent top viewing electrode, a bottom electrode and a plurality of isolated cells having well-defined size, shape and aspect ratio. The cells are filled with charged pigment particles dispersed in a dielectric solvent or solvent mixture, and a color filter is placed over the top transparent electrode.
Each of these approaches to making a full color electrophoretic display has drawbacks. For the color filter overlay approach, the color filter is typically located on top of the display device and significantly reduces the amount of incident light reflected back to the viewer, with only about 33% of the incident light being reflected back to the viewer. The result is that although color is perceived, the display is very dark in appearance. Shuttered displays require the use of lateral electrodes, and the design electronics can be very complex. See, for example, Endo et al.; Late-News Poster: Color In-Plane EPD Using an Anisotropic Scattering Layer; SID Digest Technical Papers, p. 674 (2004) and Arisawa et al.; Photo-Addressable E-Paper and Toner Display; IS&T's NIP20: International Conference on Digital Printing Technologies, Final Program and Proceedings, p. 922 (2004).
While known electrophoretic display devices, compositions and processes for displaying images are suitable for their intended purposes, a need remains for a full color electrophoretic display with a simpler, cost effective design that achieves full color and good brightness. Thus, there exists a need for a simpler, more vivid color electrophoretic device, such as a colored particle device.